• The?Doppler Effect?is defined as:

The apparent change in wavelength and frequency of a wave emitted by a moving source

• This effect can be observed in a variety of scenarios whenever sources of waves?move:
  • The?frequency?of the sound waves emitted by ambulance or police sirens goes from a?high pitch?to a?low pitch?as the vehicle whizzes past
  • Galaxies in outer space emit electromagnetic radiation - light waves - which appear to be?redder?than normal because because the stars are moving?away?from us

   

Explaining the Doppler Effect

• Usually, when a?stationary?object emits waves, the waves spread out?symmetrically

This stationary police car emits sound from the siren and the waves spread out symmetrically

• If the object emitting waves begins to?move, the waves can get?squashed?together at one end of the object, and?stretched?at the other end

To an observer in front of the moving car, the wavelength appears to be smaller because they squash together. To an observer behind the moving car, the waves appear to stretch out

• The waves at the front of the moving vehicle appear to be squashed together:
  • This means the wavelength?decreases?(and the frequency of the waves?increases)

   

• The waves behind the moving vehicle appear to be stretched out:
  • This means the wavelength?increases?(and the frequency of the waves?decreases)